The present invention relates to a multi-level metallization structure for a semiconductor device, particularly an integrated circuit, and a method of making the same. More particularly, the present invention relates to a multi-level metallization structure which uses aluminum and which provides good ohmic contact between the levels.
In the manufacture of semiconductor integrated circuits, the trend is directed toward increasing the number of components per unit area of the circuit chip in order to reduce the cost per circuit function. This increase is achieved by reducing the size of the components themselves and/or reducing the spacing between the components. However, one of the factors which limits the decrease in size and spacing is the area required by the metal interconnections used to connect the various components in a desired circuit.
One technique for overcoming the metal interconnection problem is to use multi-level metallization systems. Multi-level metallization systems include forming contact openings through the particular insulating layer which is over the substrate of the device to some of the components in the substrate. A first metal layer is deposited on the insulating layer and in the contact openings. The first metal layer is defined to form a portion of the overall metal interconnection system. A layer of insulating material is applied over the defined first metal layer and openings are formed through the insulating layer. Some of the openings can go completely through the insulating layers to some of the components in the substrate and some of the openings merely go through the second insulating layer to the defined first metal layer. A second metal layer is deposited on the second insulating layer and in the contact openings and is defined to form the remaining portion of the overall interconnection system. Thus, the defined second metal layer is connected to some of the components in the substrate and to the defined first metal layer. Additional metal layers may be used if the circuit is sufficiently complex to require them.
A metal commonly used for the metallization in an integrated circuit, because of its high conductivity, ease of application and relatively low cost is aluminum or aluminum containing a small amount of silicon. However, a problem with aluminum is that as soon as it is deposited and exposed to air, a thin layer of aluminum oxide is formed on the surface of the aluminum. Although this aluminum oxide is not a major problem when using aluminum as a single level of metallization, it is a problem for multi-level metallization. The oxide layer provides an insulating layer between the two metallization layers which creates a high resistance where the two layers contact each other. An attempt to minimize the high contact resistance by increasing the contact area has been made by making the area of the openings in the insulating layer between the metallization layers larger than the line width of the defined metallization layer. Heating the device to about 400.degree. C. to break up the aluminum oxide and sinter the two layers together further reduces the resistance. However, it has been found that this does not completely remove the oxide so that the contact resistance is still higher than if no oxide is present. Furthermore, overetching the dielectric to form the oversize contact opening causes undercutting of the dielectric beneath the metal. Undercut edges cause poor step coverage in subsequently deposited layers.